The invention relates to a method and apparatus for stuffer box crimping a multifilament yarn of the general type disclosed in EP 0 539 808 B1 and corresponding U.S. Pat. No. 5,579,566.
In conventional stuffer box crimping operations, a multifilament synthetic endless yarn is advanced into a crimping device by means of a feed nozzle. To this end, the feed nozzle comprises an injector, which introduces a pressurized fluid into a conveying channel of the feed nozzle. The crimp effect is based on heating the yarn with the use of a heated conveying medium, compacting it in a stuffer box downstream of the feed nozzle, and forming a plug thereof. The heated conveying medium is able to escape through slots provided in the wall of the stuffer box. The yarn plug is removed from the stuffer box, at a defined speed, by a delivery roll downstream of the crimping device, and subsequently cooled.
The use of the flow channel of the feed nozzle, which is disclosed in EP 0 539 808 B1 and constructed as a Laval nozzle, enables stuffer box crimping methods with yarn supply speeds as high as 4000 m/min. In particular, this increase is realized without raising the pressure of the conveying medium.
A further increase of the yarn speed requires a higher pressure of the conveying medium, since otherwise the yarn tension that is built up in the conveying nozzle will not suffice to reliably draw the yarn into the crimping device. An increased pressure of the conveying medium, however, results on the other hand in that the yarn plug is blown out of the stuffer box, since the frictional forces between the yarn plug and the stuffer box wall will no longer be adequate. Moreover, in the case of high speeds and in particular in the case of low deniers, it is no longer possible to use the feed roll downstream of the crimping device because of the risk of lap formation.
DE 21 16 274 B2 and corresponding U.S. Pat. No. 3,810,285 disclose a further device, wherein a feed nozzle and a crimping device cooperate. To this end, the conveying channel is provided directly upstream of the inlet to the stuffer box of the crimping device with a relief zone, in which the conveying fluid flowing within the conveying channel is able to expand. In this process, the yarn is bulked by loosening the filament bundle. Thus, the known device has the disadvantage that at higher yarn speeds and, with that, at higher fluid pressures, the expanded filament bundle gets hung up in the transitional region between the relief chamber and the last section of the yarn channel. As a result, an unwanted yarn plug forms in the relief chamber. A further disadvantage of the known device lies in that the yarn is deposited in the stuffer box only as a function of the conveying fluid that is carried along in the last portion of the yarn channel. A defined conveyance of the yarn into the stuffer box does not occur.
It is therefore an object of the invention to further develop the method and apparatus of the described type for stuffer box crimping a multifilament yarn such that on the one hand high yarn speeds are made possible, and that on the other hand a yarn can be produced with a high and stable crimp.
The above and other objects and advantages of the invention are achieved by the provision of a method and apparatus wherein the yarn is advanced to the crimping device in two successive conveying steps. The first conveying step is designed such that the yarn is drawn into the device at a high yarn speed. In the first step, the yarn speed is referred to as the intake speed. In the second conveying step, which immediately follows the first conveying step, the yarn already advancing at a high speed is fed into the crimping device at a conveying speed. In this process, the conveying speed in the second step is at least the same as, or greater than the intake speed of the first step. This ensures that the tension in the yarn does not fall below a minimum, so that the filament bundle remains closed between the two steps.
A further advantage of the invention lies in that the delivery of the yarn into the stuffer box can be substantially directed only to the requirements of the plug formation. The intake of the yarn for building up high yarn speeds is produced by the conveying fluid stream in the first step.
The method of the present invention is especially suitable for very high yarn speeds, with the intake speed being at least about 3,000 m/min., preferably at least about 4,000 m/min. In this process, it is preferred to withdraw the yarn from a spin zone by means of a draw system, and that after being drawn, the yarn is received by the conveying nozzle of the first step. However, it is also possible to withdraw the yarn from a feed yarn package. Regardless of the way of making the yarn available, the intake speed of the yarn in the first step needs to be directed toward the end that an adequate tension of the advancing yarn does not fall below, for example, 10 cN.
To be able to perform process variations, which are needed, for example, because of different polyester materials, the first conveying fluid stream in the first step and the second conveying fluid stream in the second step are controlled independently of each other. This permits making fine adjustments both for building up a yarn tension in the upstream yarn path and for the crimp formation in the stuffer box.
To produce the conveying fluid streams, the conveying fluid of the first step is maintained under a pressure from at least about 2 bars to at most about 15 bars, preferably from at least 4 bars to at most 12 bars. In comparison therewith the pressure of the conveying fluid for producing the second conveying stream is adjusted to a small range from at least about 1 bar to at most about 8 bars, preferably from at least 2 bars to at most 6 bars. In general, however, it is essential that the pressure of the conveying fluid in the first step assume a greater value than the pressure of the conveying fluid in the second step. In this instance, the exact adjustment values are likewise dependent on the foregoing parameters, such as, for example, the type of polymer, yarn tension, crimp formation, etc.
In an especially advantageous further development of the invention, the conveying fluid for the first conveying fluid stream and for the second conveying fluid stream is introduced by a common injector. To this end, an expansion chamber is formed between the feed nozzles, in which the outlet of the conveying channel of the first feed nozzle and the inlet of the conveying channel of the second feed nozzle terminate. The expansion chamber connects to a controllable throttle valve, so that it is possible to discharge a portion of the first conveying fluid stream that enters the expansion chamber. Thus, the portion of the first conveying stream that remains within the expansion chamber, is used for producing the second conveying fluid stream in the subsequent conveying channel. The controllable throttle valve permits adjusting the discharged conveying fluid stream.
In tests that were performed by the method of the present invention, it was possible to realize in the case of a predrawn polyester yarn of the specification 167f46 and 83f36, at yarn delivery speeds of 5,100 m/min., a crimp of 25% and a crimp stability of 85% with a yarn tension of 4 cN/dtex. The values for crimp and crimp stability were determined in accordance with DIN [German Industrial Standards] 53 840 Part 1.
In the case of a predrawn polyamide yarn of the specification 83f34, it was possible to realize at yarn delivery speeds of 4,500 m/min., a crimp of 20% and a crimp stability of 90% with a yarn tension of 4 cN/dtex.
Particularly suitable as a conveying fluid medium is hot air. It showed that the crimp of the yarn becomes greater as the hot air temperature increases. An upper limit of 180xc2x0 C. was found for polyester and 240xc2x0 C. for polyamide.
Still better crimp values are obtained with overheated, if possible, dry vapor.
To ensure a reliable plug formation in the stuffer box even at the high yarn speeds, it is advantageous to discharge the conveying fluid stream by means of a vacuum through openings in the front region of the stuffer box.
At the beginning of the process, it is possible to introduce at times a fluid through an opening in the rear region of the stuffer box against the direction of the yarn movement, and to thus exert a braking effect on the yarn, thereby initiating the plug formation in the stuffer box.
To carry out the method, the device of the present invention comprises two feed nozzles, each with a nozzle-shaped conveying channel. Both feed nozzles follow each other in the direction of the advancing yarn such that the outlet of the one conveying channel is directly opposite to the inlet of the second conveying channel. This permits producing through the feed nozzles two conveying fluid streams independently of each other for taking in the yarn in a first step, and transporting it in a second, downstream step into the stuffer box of the crimping device.
In this connection, it is possible that each of the feed nozzles has an injector for introducing one fluid stream each into the associated conveying channels. The injectors can be supplied individually or jointly by a controllable source of pressure. In the case of a joint supply from a source of pressure, means for adjusting the pressure are to be provided separately.
However, it is also possible to interconnect the feed nozzles such that they are jointly supplied by one injector.
In this connection, it will be especially advantageous, when the feed nozzles are interconnected such that an expansion chamber is formed between the conveying channels, with the outlet cross section of the conveying channel terminating in the expansion chamber being greater than the narrowest cross section of the subsequent conveying channel, so that a transportation of the conveying fluid into the subsequent conveying channel remains ensured. In this case, the expansion chamber connects to a controllable throttle valve.
To be able to produce, if possible, a high tension on the yarn as it is taken in, the conveying channel formed in the first feed nozzle, is configured preferably as a Laval nozzle, so that the conveying fluid is able to reach sonic speed in a narrowest cross section of the Laval nozzle. To ensure a uniform jamming of the yarn inside the stuffer box, the conveying channel of the second feed nozzle terminates directly in the stuffer box.
In this connection, it will be especially advantageous, when the outlet of the conveying channel and the inlet of the stuffer box are arranged in a vacuum chamber. The stuffer box connects via opening slots to the vacuum chamber, which is hooked up to a controllable source of vacuum. This further development of the invention offers a further possibility of adjusting the second conveying fluid stream.
For cooling the yarn plug leaving the stuffer box, a cooling drum is arranged preferably downstream of the crimping device. In this arrangement, the yarn plug is guided over the circumference of the cooling drum and cooled by a cooling medium, preferably cooling air.